Engine Compression Test

A discharge indication of the ammeter may be caused

b. Remove the spark plug from the cylinder at

by a faulty alternator, faulty regulator setting, or a

which compression is to be tested.

shorted or grounded lead.

c. Insert the end of the compression testing gage

into the spark plug hole and hold it there.

e. With the engine at full throttle, partially close the

d. With the battery disconnect switch and the

valve in the discharge line (if one is provided) so that

ignition switch in the ON positions, press and hold the

the indication on the discharge pressure gage is

starter pushbutton long enough to provide about five

approximately 90 psi. With the engine operating at this

compression strokes at the cylinder being tested. Note

loaded condition, check for visible and audible signs of

the indicated compression.

improper operation. Check for excessive smoke from

e. Repeat the procedure at the remaining five

the exhaust system or from the engine breather system.

cylinders.

Check for knocks, misfiring, excessive valve tappet

f. Check the noted compression readings. The

noise, backfiring or other signs of faulty operation.

readings should be fairly consistent for the six cylinders.

Check that the engine maintains governed speed at full

If one or more cylinders indicate obviously lower

load.

compression, check further to isolate the trouble.

f. If the engine fails to operate properly, and

g. Pour one teaspoonful of engine oil into the low

correction of the trouble is not within the scope of

compression cylinder and recheck compression. A

organizational maintenance, report it to direct support

continued low reading indicates a sticking or improperly

maintenance. A compression test, as described in

seating valve. If the reading now approaches normal, it

paragraph 4-51, may help to isolate and identify engine

indicates faulty piston rings, scored cylinder walls, or

problems.

worn pistons.

h. A sticking valve can sometimes be freed by

4-51. Engine Compression Test

adding a valve lubricant to the engine oil and running

the engine. Report erratic engine compression to direct

a. Ground the primary of the magneto to prevent

support maintenance.

the engine from starting.

4-46